Transimpedance amplifiers find use in many areas of electronics, since they convert a current to a voltage with a predetermined gain. One particular usage of transimpedance amplifiers is to convert the output of a photosensor, which outputs a current responsive to received light, to a voltage, thus resulting in a light sensor. Modern electronics provides large economies of scale by implementing as much circuitry as possible via integrated circuits, which can be economically mass produced.
The simplest transimpedance amplifier implemented with an operational amplifier (op-amp) is constituted of an op-amp with a feedback resistor connected between the output and the inverting input of the op-amp, with the input current to be converted further connected to the inverting input of the op-amp. The non-inverting input of the op-amp is connected to a common potential, and the output of the op-amp thus presents a voltage equal to the input current times the feedback resistor value. Unfortunately, the voltage value achievable by a simple transimpedance amplifier for the very low currents output by a photosensor diode typically requires yet further amplification, since the output of a photosensor diode which is to be amplified can be as low as a few picoamperes.
An op-amp based amplifier provides a fixed gain based on the ratio between two feedback resistors. Amplification of the output of the above described op-amp based transimpedance amplifier by an op-amp based amplifier thus results in an overall gain which is a function of the value of the resistors. Unfortunately, process variations in the production of integrated circuit based resistors result in resistor values which may be different than design values by up to 40% over a range of temperature conditions and process variations. Thus, the overall gain is not a fixed value, but is instead variable since it is responsive to the temperature conditions and process variations. It is possible to produce matched internal resistors which are physically near each other and thus respond in synchronization to die and external temperature variations, however as indicated above the overall gain of an op-amp based transimpedance amplifier followed by an op-amp based amplifier is a function of the product of two resistor values divided by a single resistor value. Thus, temperature and process variations do not cancel out.
The above can be resolved by the use of external resistors exhibiting a low temperature coefficient of resistance (TCR), however such resistors are expensive and require external terminals for connection which further significantly adds to cost.
Thus, it would be desirable to have a transimpedance amplifier arrangement exhibiting reduced temperature and process gain variation, preferably with few external components. Preferably, the transimpedance amplifier should exhibit sufficient gain for use with a photosensor so as to implement a light sensor exhibiting reduced temperature and process gain variation.